The capacity of optical transmission systems comprising fibre optics is increased by using wavelength division multiplexing (WDM) technology. For the operation and maintenance of such an optical transmission system it is necessary to monitor the signal's wavelength. In conventional optical transmission systems it is possible to use pilot tones for monitoring the WDM signals directly in an optical layer. These pilot tones form small sinusoidal signal components added to the WDM signals. Pilot tones can be used to monitor various optical parameters of a WDM signal such as optical power, wavelength and optical signal to noise ratio (OSNR). The pilot tones allow to monitor parameters of the WDM signal without expensive demultiplexing filters such as tunable optical filters and diffraction gratings. Pilot tones are suited for being used in a dynamic WDM network since pilot tones are bound to follow their corresponding optical signals anywhere in the WDM network. Therefore, an optical path taken by each WDM signal can be monitored by tracking its tone frequency. Pilot tones are sent from a first optical terminal to a second optical terminal. Pilot tones of conventional optical transmission systems are used for frequency stabilization, channel recognition or monitoring of the transported optical signals. In an optical transmission system the bandwidth between a transmitting and a receiving terminal is limited. In conventional optical transmission systems additional communication channels such as an optical supervisory channel are commonly used. However, the use of such a communication channel diminishes the remaining data bandwidth.